Detection of arsenosugars from kelp extracts via IC-electrospray ionization-MS-MS and IC membrane hydride generation ICP-MS

Abstract

The selectivity and the ability to obtain structural information from detection schemes used in arsenic speciation research are growing analytical requirements driven by the growing number of arsenicals extracted from natural products and the need to minimize misidentification in exposure assessments. Three arsenosugars were extracted from ribbon kelp utilizing accelerated solvent extraction. The three arsenosugars were separated from other arsenicals with near baseline resolution using a PRP-X100 column and a 20 mM (NH₄)₂CO₃ mobile phase at a pH of 9 with IC-ICP-MS detection. Utilizing these chromatographic conditions, the molecular weight was determined for each arsenosugar utilizing ion chromatography-electrospray ionization-mass spectrometry (IC-ESI-MS) in the positive ion mode. The molecular weight and retention times for the three arsenicals are 328 u (4.6 min), 482 u (8.2 min) and 392 u (14.2 min). The IC-ESI-MS-MS spectra from each of the arsenosugars were compared to the spectra reported in the literature, which were obtained via direct infusion of standard materials. All three MS-MS spectra contain m/z 237, 195 and 97, which are fragments of the base dimethylarsinylriboside common to all the arsenosugars. Adequate sensitivity for each arsenical was achieved using a 6.1 ng and a 22 ng injection for IC-ESI-MS and IC-ESI-MS-MS, respectively. Given the unavailability of standards, the arsenosugar distribution was determined via relative chromatographic areas using IC-ICP-MS. The IC-ICP-MS indicated the presence of an arsenic heteroatom within the same retention windows in which the arsenosugars were detected via IC-ESI-MS. The IC-ESI-MS and IC-ESI-MS-MS detection scheme provided structural information but at reduced sensitivity. In an attempt to preserve sensitivity and improve selectivity of the IC-ICP-MS, an on-line membrane hydride generation detection scheme was evaluated. The hydride system indicated that the three unknown peaks (arsenosugars) were not hydride active, thereby simplifying the chromatographic resolution needed to quantitate the more toxicologically important arsenicals, such as MMA, DMA, As(III) and As(V), while minimizing the potential for misidentification.